1. Technical Field
The present invention relates generally to spray forming methods and arrangements, and more specifically to automation of monitoring and control aspects of a spray form process.
2. Background Art
It is a known process to spray-form certain articles using moltenizing arc guns with metal wire supplied thereto. In order to moltenize the wire and form sprayable metal droplets, a significant amount of energy, typically manifest as heat, is applied at the arc gun to the wire. As a result, the temperature of the droplets is significantly elevated, and this elevated temperature is at least partially carried onward to the article being spray formed. Once the droplets land on the article and become a constituent component thereof, a portion of the heat energy travels conductively into the article, while the balance of the heat energy dissipates to the surrounding atmosphere. As a result, the temperature of the article, when considered in two and three dimensions, is often quite variable in a conventional metal spray-forming process. These variations or temperature gradients that are experienced across the body of the article during the spray-forming process can produce significant undesirable effects in the finished product.
One of the more significant detrimental effects that may occur is typically manifest as internal stress that is trapped within the substantially rigid article after its manufacture. Even though minor latent stresses may not significantly affect a finished article, it is not uncommon for stresses of magnitudes high enough to warp or otherwise cause deformation and deflection in the finished article to occur in uncontrolled spray processes. In such processes, it is not uncommon to experience temperature variations across the body of the article on the order of as much as 100xc2x0 Celsius. Still further, even minor deflections due to internalized stress can render conventional spray form processes unuseable when precision tooling is required for particular finished products or articles.
In another aspect, as the technology and processes for spray forming metallic articles advance, the manufacture of larger and larger monolithic bodies is becoming feasible. As a result, however, the temperature gradients experienced in such larger spray formed bodies is becoming more pronounced due to their greater x-, y-, and also z-dimensions. Additionally, an increased magnitude in the experienced temperature gradients will result due to the greater time required to complete these larger bodies. The thicknesses (z-dimension) of the sprayed articles will also increase in order to support the shape of the more massive bodies. Each of these characteristics contribute to the experienced temperature variations as proportionally more heat is allowed to dissipate from the body at locations distant from where the arc guns are applying heated molten metal droplets at any given point in time during the spraying process. The result can be undesirable migrating xe2x80x9chot spotsxe2x80x9d or trails across the finished product.
The detrimental effects of these experienced temperature gradients across a spray formed article have long been appreciated; not the least of which can be, and often is, the inducement of internal stresses. Still further, currently available technology provides the user with an ability to control the amount of heat energy input into the wire in the moltenizing process. But, in spite of the recognized need, a continuing failure in the art has been an inability to accurately monitor and measure the experienced temperature(s) across the article""s surface during the spray forming process on a real-time basis. Consequently, there has been a continuing inability to affect proper control over at least the heat energy input to the metal on a similar real-time basis for obviating the problems associated with temperature gradients induced in the article being spray formed.
In view of the above described deficiencies associated with unmonitored and uncontrolled spray form processes when considering temperature variations/gradients across the article being formed, the present invention has been developed to alleviate these drawbacks and provide further benefits to the user. These enhancements and benefits are described in greater detail hereinbelow with respect to illustrative embodiments of the present invention.
A new spray form cell for accommodating rapid tooling processes has been developed, primarily with the automotive industry in mind, in which a tool may be made by spray-forming molten steel onto a ceramic substrate. The molten steel is sprayed onto the ceramic substrate model that has been configured to produce a specifically shaped tool. In the instance of the manufacture of a stamping tool, the shape of the model corresponds to the article to be stamp-manufactured using the produced tool. In one embodiment, the spray is produced using a number of twin-wire arc plasma torches or guns. In an exemplarily embodiment, four such guns are utilized and their movement and performance is automated; that is, the guns are computer/robot controlled. Although most conventional thermal spray processes produce thin coatings on the order of 0.0098 inches (250 microns), this spray process is used to form much thicker deposits, for example, up to 0.24626 feet (75 mm).
During the spraying process, it is important that thermal gradients in the material be held to a minimum. That is to say, a uniform temperature is desired across the article being sprayed. In the exemplary embodiment, the article is a stamping tool suitable for use in high-production stamp-type manufacturing, such as that which is often employed in automotive manufacturing processes. Because of the relatively small size of the guns"" spray plume, compared to the size of the article or billet being spray formed, careful control of the spray pattern is required. To obtain and assure even thermal distribution across the article during the spray deposition process, real-time monitoring of the article""s temperature(s) is required.
According to the present invention, a two-wavelength imaging pyrometer is utilized to provide real-time measurement of the surface temperature distribution of a spray formed article. The imaging pyrometer provides a continuous stream of high resolution (on the order of 32,000-pixel) thermal images of the steel billet throughout the spray-forming process. The preferred imaging pyrometer, with its high sensitivity, measures temperatures as low as 392xc2x0 Fahrenheit (200xc2x0 Celsius). Through the use of two-wavelength sensing, the pyrometer is capable of making accurate surface temperature distribution measurements despite the scattering of light due to the dusty environment in the spray-forming process. Similarly, the selected pyrometer is also capable of making accurate temperature distribution measurements in spite of other opacity issues such as when the optical windows of the device become coated with dust and the degree at which light passes therethrough significantly degrades.
From an operational standpoint, the incorporation of such a real-time temperature measuring device enables control strategies that minimize or eliminate the stress-inducing characteristics of previously known processes. For instance, with an accurate, real-time, two-dimensional, temperature map of the exposed surface of the article being formed, spray gun operation and movement patterns can be altered to, among other things, minimize temperature variations across the article. From a monitoring or feed back perspective, the real-time temperature monitoring enabled by the pyrometer makes it possible to evaluate changes affected at the gun, regarding their effect on the article being sprayed.
The beneficial effects described above apply generally to the exemplary devices, mechanisms and method steps disclosed herein with regard to real-time monitoring and control of metal spray form techniques. The specific structures and steps through which these benefits are delivered will be described in greater detail hereinbelow.